Cord blood (CB) transplantation has greatly extended availability of a hematopoietic cell transplant (HCT) to patients who would not otherwise have received this curative treatment. CB has several advantages over bone marrow (BM) and mobilized peripheral blood (mPB), including ease and safety of collection, ready availability, and lessened acute and chronic graft vs. host disease (GVHD). However, a disadvantage with CB is low numbers of nucleated cells, hematopoietic progenitors (HPC), and likely stem cells (HSC), compared with BM or mPB, which has translated into increase risk of graft failure, delayed engraftment, and delayed immune reconstitution. Based on our published laboratory and preclinical animal model studies, preliminary clinical data, and mechanistic studies on CD26/Dipeptidylpeptidase (DPP) IV activities reported herein, we believe that inhibition of CD26/DPPIV will significantly enhance engraftment capability of limiting numbers of human CB cells, and accelerate time to engraftment of single CB units. This multi-PI grant proposes the following specific aims: Aim 1: Conduct a multicenter phase II clinical trial to assess efficacy and safety of systemic inhibition of CD26/DPPIV using sitagliptin, to enhance engraftment of single CB units in adults with hematological cancers. Hypothesis: Inhibition of CD26/DPPIV by systemic administration of CD26 inhibitor sitagliptin will enhance engraftment, without negatively impacting relatively lower levels of GVHD associated with single CB transplantation. Primary Objective: Evaluate proportion of patients with neutrophil recovery by day +30 after transplant as a measure of speed of engraftment. Using an optimal two-stage design, <50% of patients engrafting by day +30 will be considered unacceptable (null hypothesis; H0: p0<0.5), while 70% or more engrafting will be considered worthy of further study (alternate hypothesis: H1: p1e0.7). Subaims are: a) Evaluate time to platelet engraftment, patient survival, graft failure, relapse rate, acute and chronic GVHD levels; b) Use correlative assays to assess recovery of immune cells through phenotypic and functional analysis; and c) Assess blood levels of CD26/DPPIV, and hematopoietically relevant cytokines. Aim 2: Evaluate how CD26/DPPIV mechanistically regulates hematopoiesis, using cell culture and biochemical studies on mouse and human cytokines and with cells from mouse BM and human CB. Hypothesis: CD26/DPPIV regulates hematopoiesis through: its specific enzymatic capability to truncate and inactivate different hematopoietically active cytokines. We will compare results to clinical study in Aim 1 to better understand and utilize this new treatment modality. Subaims include: a) Evaluate how GM-CSF, G-CSF, IL-3 and EPO truncated by CD26/DPPIV, compared to full length form, signal intracellularly; b) Determine if systemic administration of sitagliptin to mice enhances homing of HSC and influences cytokine production; and c) Study other molecules for CD26/DPPIV truncation and activity.